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A clean-up operation following the takedown of what has been described as the biggest cyber-scam scam ever has begun.

Six Estonian suspects have been charged, and one Russian suspect remains at large, over a malware-based DNS changer scam that affected 4 million PCs worldwide, generating an estimated $14m in the process. The botnet – spread over 100 countries – was used to hijack browsing on infected machines in order to redirect users towards sites under the control of cyber-crooks, instead of the locations they were actually trying to visit. The technique was used to run click-fraud scams, to punt scareware at unwitting victims and to promote unlicensed pharmaceutical stores, among other scams that ran for almost five years since early 2007.

Fraudulent web pages appeared when victims attempted to visit Netflix, the US Internal Revenue Service, Apple's iTunes and other services. Infected Windows PCs and Mac machines were roped into the scam, as explained in our earlier story here.

Details of the two-year FBI-led investigation, codenamed Operation Ghost Click, were announced in New York on Wednesday after a federal indictment was unsealed. The FBI worked with the National High Tech Crime Unit of the Dutch National Police Agency on the case as well as security industry partners and academics. Trend Micro, Team Cymru, Georgia Tech University, Mandiant, Neustar, Spamhaus, University of Alabama at Birmingham and others formed the DNS Changer Working Group (DCWG) that figured out how the scam was operating and assisted law enforcement in its investigation.

Trend Micro ha published a detailed write-up of the how the scam worked from a technical perspective, and the shady firms involved, here.

As a result of the investigation, six suspected cyber-crooks were arrested in Estonia. Many are linked to Rove Digital, the Estonian firm at the centre of the probe, whose principals previously ran Esthost, an unsavoury reseller of web hosting services that was taken offline in 2008.

Botnet army commanded by 100 servers

The US has applied for extradition warrants against the six Estonian suspects, including Vladimir Tsastsin, 31, chief exec of Rove Digital. In the meantime a clean-up operation is getting under way.

US authorities seized computers and rogue DNS servers at various locations. The rogue DNS servers will be replaced by legitimate servers, a move that will mean that those infected with the malware will realise that something is wrong. The command & control (C&C) infrastructure behind the scam included more than 100 servers.

In a parallel move, Dutch police have advised RIPE (the Regional Internet Registry of Europe and the Middle East) to not change the registration of four specific blocks of IPv4 addresses until next March.

Simply swapping out DNS servers will not remove the DNSChanger malware — or other viruses it may have facilitated — from infected machines. The FBI wants DNSChanger victims to notify them about infections, a move seemingly designed to strengthen its hand in upcoming extradition proceedings against the accused.

The FBI has published an online tool designed to allow concerned punters to check if their DNS server settings have been tampered with. Advice on how to use the tool, which involves checking settings on your machine prior to entering DNS details, as well as links to Trend Micro's freebie anti-malware scanner, can be found in a blog post by Rik Ferguson here. ®